This invention relates to nuclear reactors and, more particularly, to a reactor pressure vessel for such a reactor. A major objective of the present invention is a reactor pressure vessel susceptible of more convenient and economical in-service inspection.
Boiling-water reactors store heat generated by a reactor core primarily in the form of a phase conversion of a heat transfer medium from a liquid phase to a vapor phase. The vapor phase can be used to physically transfer stored energy to a turbine that drives a generator, to produce electricity. Condensate from the turbine can be returned to the reactor, merging with recirculating liquid for further heat transfer and cooling.
BWRs typically have a reactor pressure vessel (RPV) that encloses the nuclear fuel. The RPV confines water and steam to predetermined circulation paths that are isolated from a drywell in which the RPV is located. The RPV is supported by a carbon steel skirt that flares downward and outward to a flange that rests on a concrete floor within the drywell. The length of this skirt is selected to limit stresses due to thermal and dynamic loads. In some designs, the skirt also serves as a seal between the upper and lower levels of the drywell.
An RPV must be in fluid communication with external components. Steam outlets and steam lines are provided to direct the steam to a turbine that, in turn, drives an electric generator. Feedwater inlets and conduits are provided for returning condensate from the turbine back to the RPV. For a reactor to operate safely, it is equipped with safety systems. Of particular interest herein is a gravity-driven coolant system (GDCS), which is used to replenish coolant lost from the RPV.
Such fluid communication requires the connection of conduits to the RPV. Generally, nozzles are installed on the RPV for conveniently coupling to the conduits. Typically, these nozzles are forged separately from the RPV and then welded into apertures through the RPV sidewall. Standard practice for the nuclear industry is to inspect all welds periodically.
In at least one reactor design, the nozzles for the GDCS are required to be at a height slightly below the level at which the skirt is attached to the vessel. Inspecting the welds for such nozzles would require examining under the skirt, which provides little or no clearance, or examining the welds from inside the vessel, which can be burdensome, since these welds are typically obscured by other internal components. While this problem has arisen in the context of GDCS nozzles, it can occur for nozzles used for other purposes, depending on reactor design. What is needed is an RPV that provides nozzles at support skirt level without imposing an undue burden on weld inspection.